Whenever a gas filter is used to remove or retain dangerous gases, it is important to be able to identify when the filter is exhausted and needs to be replaced. For a filter to be exhausted before being replaced would allow for maximal use of the filter but may be extremely harmful to a person who must rely on the filter for safe breathable air. By providing some indication that the filter is nearly at the end of its life, it assures the safety of a person relying upon it, but very often causes a filter to be replaced prematurely, thus contributing to added maintenance expense of a filter-breathing device.
In the case of a known warning device of this type, the nearing end of the useful life of the gas filter is indicated to the user by means of a chemical warning layer in the gas filter in such a way that an unpleasant smell is noticed by the user on the clean air side of the gas filter. This is the so-called "smell warning."
This warning device has a disadvantage, since the effect of humidity on the warning layer may incorrectly indicate prematurely the exhaustion of the gas filter retaining the dangerous gases, with the result that the gas filter is not optimally used.